Polymeric films generally are smooth and have low surface tensions due to their inherent characteristics. Printing on untreated films often results in unsatisfactory print quality due to insufficient surface wetting and insufficient ink adhesion. There is also the possibility of surface contaminants on film surfaces which can further reduce print quality.
Various coatings have been applied to substrates such as polymeric films to improve their printability. The improved ink performance of coated films may be due to improved surface tension, altered polarity, different degrees of micro-roughness, or other physical or chemical factors. Polymeric coatings can be applied as solutions, emulsions, dispersions, suspensions or 100% solid systems, by a number of methods such as roll coating, gravure coating, rod coating, and other methods known to those skilled in the art.
Ultraviolet (UV) light or electron beam (EB) curing of 100% solid systems is desirable for a number of reasons including high efficiency, high productivity and improved environmental acceptability. With 100% solid systems cured by UV or EB technology, no solvents are required, and this results in reduced pollution possibilities as well as reduced capital equipment and process costs due to the lack of solvent evaporation and recovery requirements. In addition, the absence of a solvent results in higher line speeds without the limitations of oven-drying capabilities, and curing occurs rapidly at low temperatures which reduces process effects on substrates which may be heat-sensitive. The coatings themselves generally have fewer defects and, consequently, improved properties since it is not necessary for solvent molecules to diffuse out of the coating during cure. For the reasons outlined above, space requirements, waste, and energy consumption are also lower with radiation-curable systems.
Radiation curing of polymeric systems may utilize electron beam curing or ultraviolet curing. UV curing of polymeric systems requires the presence of at least one photoinitiator whereas curing by EB techniques does not require a photoinitiator. With the exception of the presence or absence of photoinitiator, the formulations cured by either UV or EB technology may otherwise be identical.
U.S. Pat. No. 4,008,115 (Fairbanks et al) describes a method of making a series of laid-on labels each of which has a solvent and abrasion-resistant radiation-cured overcoating. The patentees describe radiation-curable liquids which may be epoxy prepolymers acrylated to provide terminal polymerizable acrylate groups, or acrylated polyether-polyisocyanate prepolymers or oligomers which may be dissolved in acrylate monomers which are copolymerizable therewith. Suitable monomers include trimethylolpropane triacrylate, 1,4-butanedioldiacrylate, neopentylglycol diacrylate, pentaerythritol tetraacrylate, 1,6-hexanedioldiacrylate, etc.
U.S. Pat. No. 4,643,730 (Chen et al) also describe radiation-curable formulations for polyethylene film reinforcement relating to disposable diapers. The patentees describe a curable coating composition which is a mixture consisting essentially of (a) from about 30% to about 60% by weight of at least one compound selected from the group consisting of urethane acrylate acrylic oligomers, acrylated acrylic oligomers and epoxy acrylate acrylic oligomers; (b) from 30% to 50% by weight of at least one compound selected from the group consisting of monofunctional acrylate monomers, difunctional acrylate monomers and acrylic monomers; and (c) about 0% to 15% by weight of trifunctional acrylate monomers with the proviso that the component materials total 100% by weight.
U.S. Pat. No. 4,942,060 (Grossa) relates to solid imaging methods utilizing photohardenable compositions of self-limiting thickness by phase separation. The photohardenable compositions described in this patent contain at least one photohardenable monomer or oligomer and at least one photoinitiator. A list of suitable monomers is found in Col. 5, line 42 to Col. 6, line 27. Included in the list of suitable monomers are triethylene glycol dimethacrylate, trimethylolpropanetriacrylate,ethoxylatedpentaerythritoltriacrylate,propox ylated neopentyl glycol diacrylate and methacrylate, and mixtures thereof.
U.S. Pat. No. 5,418,016 (Cornforth et al) describes radiation-curable compositions comprising N-vinyl formamide and an oligomer which includes epoxy-acrylate resins, polyester-acrylate resins, polyurethane-acrylate resins, acrylic acrylate resins, vinyl-ether resins, etc. The compositions are reported to be useful for a range of applications including pigmented and unpigmented coatings, printing inks, adhesives, etc.
EP Application 505 737 A1 describes UV curable coating compositions which include an acrylated aliphatic urethane in combination with a methacrylic functionalized colloidal silica and acrylic ester monomer. The coating can be applied to a thermoplastic substrate.
U.S. Pat. No. 5,436,073 (Williams et al) describes composite laminates comprising (A) a substrate sheet of paper; (B) a first coating bonded to one surface of the substrate comprising a radiation-cured acrylic composition comprising, prior to curing (i) an acrylated or methacrylated organic polyamino compound, and (ii) an acrylated or methacrylated organic polyhydroxy compound, and (C) a second coating comprising a polyolefin film bonded to the other surface of the substrate.
It is generally accepted that the use of multifunctional monomers and coatings leads to poor adhesion, and the use of monofunctional monomers leads to slow cure speeds and reduced chemical resistance and strength properties. For example, U.S. Pat. No. 5,418,016 discloses that high functionality monomers give rapid cure speeds and high cross-link density leading to films of high hardness and tensile strength with excellent chemical resistance. The films, however, suffer from reduced adhesion. Monofunctional monomers, conversely, give slow cure speeds and low cross-link density resulting in cured films of lower hardness, tensile strength and with reduced chemical resistance.